The difference that linear energy transfer makes to precursors of DNA strand breaks.

Using agarose gel electrophoresis, we have measured the yields of DN A single- and double-strand breaks (SSBs and DSBs) for plasmid DNA irradiated in aerobic aqueous solution with either 137Cs gamma rays or 4He ions with a mean LET of 94 or 150 keV micron-1. The presence of dimethyl sulfoxide (DMSO) resulted in a decrease in the yields of both SSBs and DSBs, with a greater decrease being apparent for gamma irradiation than for 4He-ion irradiation. Irradiation by 4He ions in the presence of N-(2-thioethyl)- 1,3-diaminopropane (WR-1065) resulted in a decrease in the yield of SSBs and a slightly larger decrease in the yield of DSBs. Together with results obtained previously, these observations suggest a substantial contribution to the formation of SSBs and DSBs by 4He ions by species containing at least two radicals and more than two radicals, respectively.